Communications system employing single-mode lasers and multimode optical fibers

ABSTRACT

An optical transmission system having a single mode laser that generates an optical signal that is carried by a multimode optical fiber to a receiver is disclosed. The single mode laser has an emitting aperture from which the optical signal is routed to the input end of the multimode optical fiber. The receiver receives light from the output end of the optical fiber. The receiver includes an equalizer that corrects the received light for modal dispersion introduced by the multimode optical fiber. Light leaving the emitting aperture of the laser is introduced into the multimode optical fiber in a pattern that excites a subset of the plurality of optical transmission modes thereby reducing the modal dispersion introduced into the light signal and stabilizing the dispersion in time. The improved dispersion enables further correction of the dispersion through the utilization of equalization techniques.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/394,840 filed Mar. 21, 2003, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The transmission of electronic signals by converting the signals tooptical signals that are transmitted via optical fibers has significantadvantages over using metallic conductors to transmit the electronicsignals. Optical fibers have higher bandwidth, and hence, can carry moredata per unit time. In addition, optical fibers have reduced noise andare less expensive than copper conductors.

Signals are transmitted on optical fibers by first converting theelectrical signals to an optical signal using a light converter such asa laser or an LED. The optical signal is then coupled into the opticalfiber transmission line that may include a number of amplificationstations. At the receiving end of the optical fiber, the optical signalis converted back into an electrical signal.

Light traveling down the optical fiber is dispersed in time. The timedispersion is the result of the range in wavelengths generated by thelight conversion device and/or the different optical paths through theoptical fiber. The dispersion effects can be corrected to some degree bythe use of equalization techniques provided the properties of thetransmission system are stable over a sufficient period of time. Inprinciple, if the distortion properties of the optical fiber are known,a filter can be provided at the receiver that corrects for thedistortion. Some examples of such equalization are described in the textbook, E. A. Lee et al., Digital Communication, Kulwer AcademicPublishers (1988). Adaptive equalization utilizes equalization that isadjusted while signals are being transmitted in order to adapt tochanging line characteristics. However, even with adaptive equalization,the properties of the communication link must remain constant over atime period that is long compared to the time needed to transmit one bitof information.

To maximize the amount of information that can be transmitted on afiber, both the total dispersion and the variations in the dispersionover time need to be minimized. This dispersion limits the distance overwhich the signals can be transmitted. Dispersion is introduced both bythe light source and the transmission fibers. Single mode lasersgenerate a very small wavelength range, which results in the lighttraveling down the fiber with a very small spread in transmission timesprovided all of the light traverses the same path through the fiber. Inprinciple, a transmission system that utilizes single-mode fibers thatare driven by single mode lasers has the least dispersion; however,single mode fibers present additional problems that discourage suchuses, especially for low-cost applications

There are two types of optical fibers, single-mode optical fibers (SMFs)and multi-mode optical fibers (MMFs). Single-mode fibers provide aninherently less dispersive transmission path; however, such fiberspresent other problems. Compared with a multimode fiber, a single-modefiber has higher bandwidth and can carry signals for longer distancesdue to the reduced signal dispersion. Also, since a single-mode fiberonly has one mode there is no modal dispersion, i.e., all of the lighttraverses the same optical path. However, it is more expensive tomanufacture fiber optic modules for single-mode fibers due, at least inpart, to the tighter alignment requirements between the light source andthe optical fiber.

Multimode fibers are large enough in diameter to allow low cost lightsources such as light-emitting diodes (LEDs) and vertical-cavitysurface-emitting lasers (VCSELs) to be coupled into the fiber utilizinglow cost assembly methods. More expensive single-mode lasers may also becoupled into a multimode fiber. However, when light rays from multimodeor single-mode lasers are directly coupled into a MMF, the light raystravel through multiple path lengths (zigzag with varying numbers ofbounces from the walls of the fiber) through the MMF, causing signal ormodal dispersion. The specific path or mode taken by any given light raydepends on the position and angle of incidence on the end of the fiberat which the light ray enters. This modal dispersion has limited thetransmission distance of a multimode fiber compared to a single-modefiber. For this reason, MMFs have generally been employed to transmitlight signals from sources such as VCSELs only for short distances,typically less than 300 meters.

Modal dispersion may, in part, be compensated for with the use ofequalization techniques on the fiber optic link at the receiver.Adaptive equalization utilizes equalization that is adjusted whilesignals are being transmitted in order to adapt to changing linecharacteristics. Adaptive equalization introduces components to ananalog or digital circuit to compensate for signal attenuation and delaydistortion in the transmission system as a function of frequency. Insuch systems, the transmission link is periodically examined todetermine the distortions introduced in to the link by using a signal ofknown composition. Once the distortions are determined, a “filter” canbe introduced into the receiver. The filter introduces the inversedistortions into the received signal, and hence, corrects for the knowndistortions of the communication link. For this type of strategy tosucceed, the distortions introduced by the communication link mustchange slowly compared to the update rate of the filter.

The distortion introduced by modal dispersion changes rapidly with time,and hence, MMFs have not been good candidates for equalizationtechniques. The precise modes that are excited when a single mode laseris coupled to a MMF depend on the coupling mechanism that is employedand the stability of the mode and output spot on the laser face. Smallchanges in the coupling conditions can result in very large changes inthe specific modes that are excited, and these changes can take place atrates that approach the bit rate of the transmission link.

SUMMARY OF THE INVENTION

The present invention includes an optical transmission system having asingle mode laser that generates an optical signal that is carried by amultimode optical fiber to a receiver. The single mode laser has anemitting aperture from which the optical signal is routed to the inputend of the multimode optical fiber. The receiver receives light from theoutput end of the optical fiber. The receiver includes an equalizer thatcorrects the received light for modal dispersion introduced by themultimode optical fiber. Light leaving the emitting aperture of thelaser is introduced into the multimode optical fiber in a pattern thatexcites a subset of the plurality of optical transmission modes therebyreducing the modal dispersion introduced into the light signal andstabilizing the dispersion in time. The improved dispersion enablesfurther correction of the dispersion through the utilization ofequalization techniques. The light from the emitting aperture can berouted to the input end of the optical fiber by an optical element thatprovides a non-uniform pattern of illumination over the input end of theoptical fiber.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A is a block diagram of an optical communication system accordingto one embodiment of the present invention.

FIG. 1B illustrates one embodiment of an input apparatus according tothe present invention for coupling a signal from a laser to a MMF.

FIG. 2A illustrates another embodiment of an input apparatus accordingto the present invention for coupling a signal from a laser to a MMF.

FIG. 2B is a front view of an offset patch cord.

FIG. 3 is a side view showing laser offset with respect to thecenterline of a MMF.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is based on the observation that even with a MMF,a light signal can be launched into the MMF in a manner that limits thenumber of modes that are excited in the MMF. This restricted set ofmodes is more stable in time, and hence, the dispersion characteristicsof the MMF over time are also substantially more stable than thedispersion characteristics obtained with a conventional launch of asingle-mode light signal into a MMF. This increase in stability makesadaptive dispersion correction possible, and hence, provides a means forimplementing a long distance MMF connection that makes use of the largeinstalled base of MMF channels.

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIG. 1A, which is a blockdiagram of an optical communication system 10 according to oneembodiment of the present invention. Communication system 10 converts aninput signal to a light signal via a laser 11. The output of laser 11 islaunched into a MMF 13 with the aid of a conditioning optical element12. The light signal is converted back to an electrical signal by areceiver 14 that includes an equalization circuit that corrects fordispersion of the signal in MMF 13. The equalization circuit utilizesgain parameters that are computed by comparing the signal output fromthe receiver in the absence of equalization with the input to the laserwhen a known training signal is sent over the communication system.

A controller 15 can be utilized to compute the gain parameters thatbring the output signal as close to the input signal as possible for theequalization algorithm employed by the receiver/equalizer 14. Theseparameters can be determined periodically if the communication linkchanges over time. As noted above, for this strategy to operatesuccessfully, the parameters must remain constant over a time periodthat is long compared to the time period required to send a single bitof information over the MMF. In particular, the parameters must remainconstant between calibrations. The present invention provides atransmission environment that is sufficiently stable to allow suchequalization.

The typical laser is a laser having a single mode such as a single-modeVCSEL or a Fabry-Perot laser; although other single mode lasers can beutilized. The output of the laser can be modulated by an external lightmodulator that responds to the input signal or by modulating theelectrical signal across the gain medium within the laser. Theadvantages of the present invention can be realized with any suchlasers.

The present invention utilizes a conditioning optical element 12 tolimit the modes of the MMF that are excited. The conditioning opticalelement is a connection block between laser 11 and multimode fiber 13.The conditioning optical element transmits a conditioned light sourcedata signal into the MMF thereby launching the signal into a restrictedset of modes of the MMF. The use of a single-mode laser in combinationwith the conditioning optical element improves the stability of thelaunch as compared to the use of multimode lasers, or even a single-modelaser without conditioning. MMFs are commonly found in present dayinstallations, and the present invention takes advantage of theinstalled base.

The present invention provides an improved bandwidth-distance product asa result of employment of a single mode laser and MMF combination with aconditioned launch, to enable effective use of adaptive equalizationtechniques, which is possible due to the more stable signal impulseresponse resulting from use of the invention. The usual combination of alaser with multiple modes when launched into a MMF with loose alignmenttolerances will result in an impulse response that may vary rapidly withtime. This resulting impulse response will be due to variations in thespatial modal profile of the laser itself, variations in coupling to thevarious modes of the MMF, and transmission variations through the MMF.In a severe case, the resulting impulse response may vary on a timescale similar to the bit rate, making it very difficult to implementadaptive equalization techniques as the equalization would need to adapton a bit by bit time basis. The modal dispersion will result in severereceiver errors for some link lengths.

In one exemplary embodiment of the present invention a diffractiveoptical element (DOE) is placed between a single-mode laser and the MMF.This DOE functions to transform the incoming light from the laser into aspecific shaped (for example, a doughnut shape or some other uniformshape) light signal that launches into the MMF such that it excites arestricted set of modes in the MMF.

In another exemplary embodiment an offset patch cord is utilized. Thepatch cord is placed between the single-mode laser and the MMF. Thepatch cord consists of a SMF and a MMF piece. The laser is aligned intothe SMF input portion of the offset patch cord, the SMF is, in turn,coupled with a fixed offset from the center of the core of the MMFportion of the offset patch cord, and then the MMF output of the offsetpatch cord is connected to the desired MMF link.

In yet another exemplary embodiment the laser is directly offsetrelative to the MMF via alignment, thereby serving the same function asan offset patch cord. That is, the end of the MMF is illuminated with aspot of light that is smaller than the diameter of the MMF and which isoffset such that only a restricted set of modes is excited.

In one embodiment, a single-mode VCSEL is used to convert an initialoutput light source data signal into a conditioned launch, using one ofthe above techniques for achieving the desired conditioning. Theconditioned launch functions to condition the output light source datasignal from the VCSEL into a light core (MMF) so that a more stable andrestricted set of modes are excited in the fiber. The conditioned launchrestricts the set of modes that are excited in the fiber and results inreduced modal dispersion. When used in conjunction with a single-modeVCSEL, the conditioned launch results in a very time-stable opticalpulse. The result of this method of using the VCSEL and conditionedlaunch MMF combination results in a greatly improved stabilization ofthe impulse response of the fiber over time. The resulting opticaloutput light source data signal from the MMF has reduced dispersion whencompared to the optical output of a standard multimode VCSEL launchedinto a MMF with no attempt to condition the launch. This results in anoutput that is more stable with time and has less dispersion due to thelaunch into a restricted set of MMF modes. As a result, the impulseresponse of the MMF will change on a slow time scale relative to the bitrate. Thus the impulse response will be time invariant on the time scaleof interest.

As noted above, this slow time scale change enables adaptiveequalization techniques to now be applied to the fiber optic link. Thus,the MMF output signal can be converted into an electronic output signaland adaptive equalization techniques can be applied to this signal toresult in an output electronic signal that is compensated for modaldispersion. The ability to apply adaptive equalization techniques is adirect result of the reduction of time variation achieved by using asingle mode laser, conditioned launch, and MMF combination.

One benefit of the present invention is an improved bandwidth-distanceproduct as a result of employment of the single-mode laser-MMFcombination, conditioned launch, and adaptive equalization techniques. Ahigher bandwidth-distance product allows sending a faster signal overthe same distance or increasing a distance for a set signal bandwidth.An improvement of about ten-fold or more, when compared to prior art, isexpected with the use of the apparatus and method of the presentinvention.

Yet another benefit of the present invention is that it takes advantageof the existing fiber optic installation base, which commonly has MMFinstalled. Hence, the capacity of this installed base is effectivelyincreased without replacing the existing MMFs.

Referring now to the drawing, FIG. 1B, which illustrates one embodimentof an input apparatus according to the present invention for coupling asignal from a laser 400 to a MMF 404, along with input and output lightintensities. Single mode laser 400 launches optical input 401, which isconditioned by diffractive optical element (DOE) 402 placed between thelaser and MMF 404. DOE 402 conditions optical input 401 by transformingit into a specific shape (for example, a doughnut shape or other uniformshape) that launches the signal into the center core or channel 405 ofMMF 404 such that it would excite a restricted set of modes in the MMF.The controlled launch condition provided by the DOE, and the restrictedset of modes in MMF 404, results in a very time-stable optical output406. The resulting optical output 406, as it exits core 405, has littledispersion, as well as being time stable.

The use of a single mode laser with DOE 402 thus provides a much morestable optical output as it exits from MMF 404 as compared to the use ofa multi mode laser launched into a MMF. As stated above, this allows useof adaptive equalization techniques as shown in FIG. 1A to furtherimprove the output signal.

The input apparatus shown in FIG. 2A is similar to that shown in FIG. 1Bwith the exception that offset patch cord 505 replaces DOE 402. Theoffset patch cord includes a single-mode fiber (SMF) element 503 and MMFelement 502. Offset patch cord 505 conditions the launch of opticalinput 401 from single mode laser 400 into channel 405 of MMF link 404,enabling further improvement in the stability of the optical output asit exits the MMF link. As with the FIG. 1B embodiment, the controlledlaunch thereby excites a restricted set of optical modes of MMF fibercore 405. The offset patch cord creates a launch that restricts the setof modes that are excited in MMF 404 and results in a very time-stableoptical output 406.

FIG. 2B is a front view of offset patch cord 505 showing SMF element 503and MMF element 502. Referring to FIG. 2A, laser 400 is aligned with SMF503 whereas MMF element 502 is coupled to and aligned with MMF 404 withconnecting collar 504.

Refer now to FIG. 3, which is a side view showing laser 400 offset withrespect to the centerline of MMF 404. This use of directly offsettinglaser 400 into MMF 404 does not use an optical element or an offsetpatch cord. The result, however, is to excite a restricted set of modesof MMF 404 with resulting optical output 406 having time stabilizationand low dispersion, to enable the application of adaptive equalizationtechniques.

Although the present invention has been described with reference topreferred embodiments, numerous modifications and variations can be madeand still the result will come within the scope of the invention. Nolimitation with respect to the specific embodiments disclosed herein isintended or should be inferred.

1. An optical transmission system comprising: a single mode laser having an emitting aperture; a multimode optical fiber having an input end and an output end and a plurality of optical transmission modes; and a receiver that receives light from said output end, said receiver comprising an equalizer that corrects said received light for modal dispersion introduced by said multimode optical fiber, wherein light leaving said emitting aperture of said laser is introduced into said multimode optical fiber in a pattern that excites a subset of said plurality of optical transmission modes.
 2. The optical transmission system of claim 1 further comprising an optical element that couples light from said emitting aperture to said input end.
 3. The optical transmission system of claim 2 wherein said optical element comprises a diffractive optical element that generates a non-uniform illumination pattern on said input end.
 4. The optical transmission system of claim 2 wherein said optical element comprises a patch cord that connects said emitting aperture to said input end such that said emitting aperture illuminates an off-center spot on said input end.
 5. The optical transmission system of claim 1 wherein said emitting aperture is positioned relative to said input end such that light from said emitting aperture illuminates an off-center spot on said input end.
 6. The optical transmission system of claim 1 wherein said equalizer is an adaptive equalizer.
 7. The optical transmission system of claim 1 wherein said laser comprises a VCSEL.
 8. The optical transmission system of claim 1 wherein said laser comprises a Fabry-Perot laser.
 9. The optical transmission system of claim 1 wherein said laser comprises an edge-emitting laser.
 10. The optical transmission system of claim 1 wherein said laser comprises an externally-modulated laser.
 11. A method for transmitting a signal over a MMF optical link having an input end and an output end, said method comprising: modulating a light signal from a single laser; coupling said modulated signal to said input end in a manner that excites a restricted subset of optical modes of said MMF; and receiving a light signal from said output end and correcting said received signal for dispersion in said MMF to create an output signal.
 12. The method of claim 11 wherein said modulated signal is coupled to said input end utilizing an optical element that couples light from an emitting aperture of said laser to said input end to provide a non-uniform illumination pattern.
 13. The method of claim 12 wherein said optical element comprises a diffractive optical element.
 14. The method of claim 12 wherein said optical element comprises a patch cord that connects said emitting aperture to said input end such that said emitting aperture illuminates an off-center spot on said input end.
 15. The method of claim 11 wherein said emitting aperture is positioned relative to said input end such that light from said emitting aperture illuminates an off-center spot on said input end.
 16. The method of claim 11 wherein said correction of said received signal comprises adaptive equalization. 